In recent years, power modules are used so as to control a large current and a large voltage in electrical vehicles and electrical trains from the viewpoints of improvement of performances of industrial devices and problems of the global environment. The heat produced by semiconductor devices that are mounted on them has been increasing steadily. Therefore, a heat releasing property is important in circuit boards for power modules. Ceramic-metal bonded circuit boards using a high-thermal-conduction ceramic substrate bonded to a metal plate of copper, aluminum or the like are widely used.
For high-thermal-conduction ceramic substrates, substrates of silicon nitride or aluminum nitride having a high thermal conduction property and high electrical insulation property are used. A ceramic-metal circuit boards including the high-thermal-conduction ceramic substrate bonded to a metal plate with an active brazing metal comprising Ag—Cu are widely used. As the metal plate, copper, which is superior to aluminum in electrical conductivity and thermal conduction property, is generally used. Since copper has a higher yield stress than does aluminum and differs greatly from ceramic in thermal expansion, there is a problem that the thermal cycle resistance and thermal shock resistance of the ceramic-metal circuit board are decreased and cracks are easily produced in the ceramic substrate as the thickness of the copper plate increases, which leads to a decrease in reliability.
The above-mentioned problems were reported in Patent Literatures 1, 2 and the like. In Patent Literatures 1 and 2, reliability is improved by relaxing stress concentration at the end surface portion of a copper circuit plate in the ceramic substrate by making a brazing material layer protrude outward from the copper circuit plate.
In a ceramic-metal circuit board, a predetermined circuit pattern is formed on a copper circuit plate so as to mount and connect a semiconductor device. High-definition is required for the position and form of the circuit pattern, and in order to obtain a high-definition pattern, a method comprising bonding a copper plate to a ceramic substrate and thereafter forming a circuit pattern by an etching process is generally used.